The original low pass filter converts PWM to DC voltage.
You can see the nasty ripple on the ouptut in the oscillscope:
What exactly is happening with the Sallen-Key that makes it have less ripple?
Try to explain it by identifing the effect that causes the basic low pass filter to have a downward ripple on the falling edge of the PWM signal. Is it due to a capacitor C1 discharging?? If it is due to C1 where is the current going when the PWM is on a falling edge/ 0 volts?